This invention relates to an improved process for producing cumene from benzene and propylene in the presence of an alkylation catalyst. It also relates to transalkylating di- and triisopropylbenzene with benzene in the presence of a transalkylation catalyst to produce cumene.
The present invention is broadly applicable to the production of alkylated aromatic hydrocarbons. These compounds are useful in themselves and more frequently in subsequent chemical synthesis of other compounds. The present invention is particularly applicable to the production of cumene, or isopropylbenzene, which is a reactant finding utility in the preparation of phenol, acetone, alphamethylstyrene and acetophenone. Another application of the process of the present invention may be found in the preparation of p-cymene which may be oxidized to produce p-cresol. A further application of the process is in the alkylation of a substituted aromatic compound such as phenol, which when alkylated with isobutylene forms o-tertiary-butylphenol and p-tertiary-butylphenol, both of which find utility in the resin field.
As above stated, the present invention finds particular application in the preparation of cumene. In the usual commercial process for the production of cumene, it is the practice to charge liquid benzene and liquid propylene into a reactor and to react the same therein in one or more alkylation zones in contact with an alkylation catalyst. In order to minimize the production of dialkylated products of benzene it has been the practice to maintain a molar excess of benzene throughout the reaction zone ranging from about 4:1 to about 16:1, and more preferably about 8:1 of benzene to propylene. Two competing reactions with the desired production of isopropylbenzene have created problems in the prior commercial processes used. One of these as indicated above has been the formation of dialkylated benzenes such as di- and triisopropylbenzene rather than the desired monoalkylated product. This competing reaction has been controlled by means of employing large molar excesses of benzene as indicated above. The other competing reaction causing losses in the yield of cumene based on propylene reactant charged is the formation of oligomers of propylene such as propylene dimer and trimer which occur to a limited extent even with the large molar excesses of benzene present. Propylene trimers and some of the propylene tetramers boil with cumene. Since the presence of these olefins interfere with the oxidation reaction used to make phenol from cumene, this oligomerization side reaction must be minimized to make a high purity product.
The alkylation reaction of the alkylatable aromatic compound is exothermic in nature and the temperature within the reactor tends to increase at a rapid rate. This increase in temperature caused by the exothermic reaction likewise tends to increase the production of cumeme bottoms products by the competing reactions. In the past it has been customary to control the temperature rise by catalyzing the reaction in multiple separate zones and employing a quenching medium between each of several successive alkylation zones. This quenching has served to control the temperature at which the reaction mixture enters each successive zone and thus the temperature rise throughout each zone. The temperature rise from inlet to outlet of the reactor has also been controlled by controlling the molar excess of benzene charged to the reactor, the benzene acting as a heat sink to absorb heat released by the alkylation reaction. Accordingly, increasing the molar excess of benzene charged to the reactor, with a corresponding dilution of the propylene reactant therein, not only provides more aromatic sites subject to alkylation and a resulting reduction in oligomers and over alkylated by-products, but also reduces the formation of undesirable by-products resulting from an excessive temperature rise across an alkylation zone or zones.
To obtain the desired high molar excess of benzene in the reactor charge, it has been the practice to separate the reaction zone effluent to obtain a benzene-rich stream suitable to recycle to the reactor. Since the two principal components of the reaction zone effluent are benzene and cumene, it is necessary to make a separation of the benzene and cumene, the latter being the higher boiling component. Consequently, to obtain a purified stream of benzene relatively free of cumene and suitable for recycling to the reaction zone, benzene is vaporized and fractionated, thus requiring consumption of substantial heat to vaporize benzene and provide adequate reflux in a benzene fractionator, the heat requirement being substantially proportional to the ratio of benzene to propylene desired in the charge to the reactor. At the present time, relatively high fuel cost necessitates review of processes requiring high utility consumption with the result that alternative processing schemes which previously were unattractive are becoming more desirable if utility consumption is reduced.